Update RetinaCluster algorithms
This MR adds minor improvements to Retina clustering algorithms.
Gaudi::Accumulators are introduced in VPRetinaClusterDecoder to count different types of clusters.
A new RawBank for mixed SPs is also added.
For an overview, see these slides from January 2021.
Related to Rec!2337 (merged) and Moore!733 (merged)
Edited by Rosen Matev